Nowadays a particulate filter for capture of particulates in exhaust gas is incorporated in an exhaust pipe, and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added at a position between the selective reduction catalyst and the particulate filter to thereby concurrently reduce the particulates and NOx.
In such case, the addition of the urea water to the selective reduction catalyst is at between the particulate filter and the selective reduction catalyst. In order to ensure ample reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, required is elongation of a distance between the urea water added position and the selective reduction catalyst. However, to arrange the particulate filter and the selective reduction catalyst in a sufficient spaced-apart relationship will extremely impair mountability on a vehicle.
Thus, the applicant of the invention devises a compact-sized exhaust emission control device with a particulate filter and a selective reduction catalyst arranged in parallel with each other. A discharge end of the particulate filter is connected to an entry end of the selective reduction catalyst through an S-shaped communication passage so as to make the exhaust gas from the discharge end of the particulate filter reversely turn around into the entry end of the adjacent selective reduction catalyst.
In such exhaust emission control device, the particulate filter and the selective reduction catalyst are encased by a casing which serves also as muffler. The selective reduction catalyst arranged downstream of the particulate filter has a discharge end with a tailpipe for discharge of the depurated exhaust gas outside of a vehicle.
Meanwhile, recently obliged in countries is equipment of an onboard diagnosis device (OBD) which monitors any failure in an exhaust gas emission control device and, upon failure occurrence, turns on a warning light or the like for announcement of the failure occurrence to a driver and records details of the failure. As shown in FIGS. 1-3, mounted at a deep position of this kind of tailpipe 1 is a sensor 2 for concentration-detection of particulates or particulate matter (PM) to ascertain whether the particulates are properly reduced or not.
There exists, for example, below-mentioned Patent Literature 1 showing state-of-art technology pertinent to this kind of tailpipe.